Background Theory > Spice Simulation > Devices > Bipolar Junction Transistors

In SPICE (Simulation Program with Integrated Circuit Emphasis), a BJT (Bipolar Junction Transistor) model is used to simulate the behavior of bipolar transistors, which are semiconductor devices commonly used in electronic circuits for amplification and switching. BJT models in SPICE describe the electrical characteristics of the transistor, including its current-voltage relationships, capacitances, and other parameters that affect its operation.

There are various BJT models available in SPICE, each with its level of complexity and accuracy. Some of the most commonly used BJT models in SPICE include:

Ebers-Moll Model

This is a basic and widely used model that represents the BJT as a current-controlled current source. It captures the fundamental characteristics of a bipolar transistor, including the relationships between base current, collector current, and emitter current.

The Ebers-Moll model is a simplified but fundamental model used to describe the behavior of bipolar junction transistors (BJTs) in electronics. It provides a basic representation of how current flows through the various regions of a BJT: the emitter, base, and collector. The Ebers-Moll model is widely used in SPICE simulations and other electronic design tools to analyze and predict the behavior of bipolar transistors.

The model is named after its developers, John S. Ebers and John L. Moll, who introduced it in the early 1950s. It focuses on the relationships between currents and voltages in a BJT and is based on the following key assumptions:

The BJT operates in the active region, where both the emitter-base junction and the collector-base junction are forward-biased.

The BJT operates in a small-signal AC regime, where variations in current and voltage are small around a quiescent (DC) operating point.

The Ebers-Moll model provides insights into the relationships between the voltages and currents in a BJT. It's a good starting point for understanding BJT behavior and can be useful for basic analysis, such as gain calculations and determining bias points. However, the Ebers-Moll model has limitations and does not capture all the complexities of real-world BJT behavior, such as high-frequency effects, parasitic capacitances, and other non-ideal characteristics.

For more accurate and detailed simulations, higher-level BJT models like the Gummel-Poon model or advanced models are used. These models take into account additional parameters and effects for more precise analysis of transistor behavior.

Gummel-Poon Model

The Gummel-Poon model is an extension of the Ebers-Moll model and is a more comprehensive and accurate representation of bipolar junction transistors (BJTs) in electronic circuit simulations. Named after its developers, Hermann Gummel and John Poon, this model includes additional parameters and effects to better describe the behavior of BJTs in various operating conditions.

The Gummel-Poon model enhances the Ebers-Moll model by accounting for several factors that affect BJT operation, such as:

•High-Frequency Effects: The Gummel-Poon model introduces capacitances and transit time effects, which are important in high-frequency operation. These effects are not captured by the basic Ebers-Moll model.

•Early Voltage : The Early voltage is included in the Gummel-Poon model to account for the Early effect, where an increase in collector current leads to a reduction in the effective base width. This effect affects the output characteristics of the transistor.

•Base Resistance: The Gummel-Poon model includes a base resistance term to account for the resistive nature of the base region. This resistance affects the input characteristics and voltage-divider action in the BJT.

•Temperature Effects: The Gummel-Poon model includes parameters that account for the temperature dependence of various transistor parameters, which is essential for accurate simulations over a range of temperatures.

•Emitter Saturation Current : The Gummel-Poon model introduces the emitter saturation current, which is more accurate than the Is parameter in the Ebers-Moll model.

•Additional Model Parameters: The Gummel-Poon model includes additional parameters to capture various non-ideal effects, making it suitable for a wider range of BJT applications.

The Gummel-Poon model provides a more realistic representation of BJT behavior and is particularly useful for analyzing BJT performance in various biasing and operational conditions, including high-frequency operation. However, it is also more complex and requires more parameter values to be accurately defined compared to the Ebers-Moll model.

Modified Gummel-Poon Model (MGP)

The Modified Gummel-Poon (MGP) model is an advanced extension of the Gummel-Poon model for simulating bipolar junction transistors (BJTs) in electronic circuit designs. Like the Gummel-Poon model, the MGP model includes additional parameters and effects to better capture the behavior of BJTs under various operating conditions. The MGP model further refines and enhances the accuracy of the simulation results compared to the Gummel-Poon model.

The key improvements introduced by the Modified Gummel-Poon model include:

•Improved Temperature Dependencies: The MGP model includes more accurate temperature dependencies for various parameters, making it suitable for simulations over a wide range of temperatures.

•More Detailed Transit Time Modeling: The transit time effects in the MGP model are modeled with increased accuracy, accounting for the transit time variations with collector current and voltage.

•Advanced Base Resistance Modeling: The MGP model introduces a more sophisticated representation of the base resistance and its impact on transistor performance.

•Enhanced Early Voltage Model: The Early voltage model in the MGP model is designed to provide better accuracy over a wider range of operating conditions.

•Bias-Dependent Current Sources: The MGP model includes bias-dependent current sources that consider the impact of operating conditions on transistor behavior.

•Higher Accuracy at High Frequencies: The MGP model provides improved accuracy in high-frequency simulations compared to the standard Gummel-Poon model.

The Modified Gummel-Poon model addresses some of the limitations and inaccuracies associated with the Gummel-Poon model, especially in scenarios involving temperature variations, high-frequency operation, and complex biasing conditions. It is a suitable choice for accurate simulations of BJTs in various electronic circuits, including analog amplifiers, RF circuits, and mixed-signal designs.

It's important to note that as the complexity of the model increases, so does the number of parameters that need to be accurately characterized for specific transistors. Therefore, careful attention to model parameter extraction and verification is crucial for obtaining reliable simulation results.

Designers often select the appropriate level of BJT model based on the complexity of their design, the accuracy required, and the level of detail necessary to capture the transistor's behavior under different conditions

Level-1, Level-2, Level-3, etc. Models

These models are increasingly complex and accurate, incorporating more parameters to represent various aspects of transistor behavior in greater detail. Higher-level models consider effects like parasitic capacitances, transit time effects, and temperature dependencies.

When using a BJT model in SPICE simulation, you typically need to provide parameters that define the transistor's characteristics, such as doping concentrations, junction area, and transit times. The accuracy of the simulation depends on the choice of the model and the accuracy of the provided parameters.

To simulate a circuit containing BJTs in SPICE, you generally need to:

Include the appropriate BJT model in your SPICE net-list.

Specify the model parameters based on the specific transistor you are using.

Connect the transistor in the circuit with proper biasing and connections.

Set up your simulation (DC analysis, AC analysis, transient analysis, etc.).

Run the simulation to observe the behavior of the BJT and the entire circuit.

Keep in mind that BJT models are part of a larger SPICE simulation setup, and the accuracy of the simulation depends on the quality of the model, the chosen simulation settings, and the accuracy of the provided transistor parameters. It's important to refer to the documentation of your specific SPICE software for information on BJT models and their usage.